From Consumer Wiki

ORGAN DONATION

Individuals interested in donating their body organs upon death can obtain an organ donor card through the California Department of Motor Vehicles. Although the donor card is a legal document, a family member can override the donor's decision. The only legal or binding document for organ donation is the "Durable Power of Attorney for Health Care". This document is given to a trusted individual who will carry out the wishes of the donor and abide by his or her decision. If neither document exists, it becomes the decision of the next of kin whether the organs will be donated.